The present invention relates in general to telecommunication systems and, more particularly, to reducing sidetone in the receive signal path of a telecommunication system.
Telecommunication systems having transmit and receive signals paths are commonly used in applications such as cordless telephones, cellular telephones, corded telephones, and many other two-way communication devices. Cordless telephones for example are widely used in residential and commercial environments because of their flexibility in allowing the user to venture some distance from the base unit during the conversation. The near party speaks into the handset microphone and the voice data is transmitted by RF link to the base unit. The base unit sends the voice data across hard-wire telephone lines to the far party on the other end. Voice signals from the far party are received from the hard-wire telephone lines by the base unit and transmitted over the RF link to the handset to complete the two-way conversation.
When the near party speaks into the microphone, a sidetone signal is typically generated between the microphone and earpiece so that the near party can hear his/her own voice. Some sidetone in the earpiece is desirable so that the near talker does not believe the telephone line to be dead while the far party is listening. On the other hand, the sidetone should not overwhelm or interfere with the far party's voice when both parties are speaking at once.
The conventional interface for cordless telephones is a four-wire to two-wire conversion circuit that allows the transmit and receive signals of the base unit to be interfaced with the two-wire telephone line. The four-wire to two-wire interface typically has impedance mismatches that cause reflections of voice signals back in the transmit path of the base unit. The reflections increase sidetone in the near party's earpiece. If the sidetone volume becomes sufficiently loud it may interfere with the far party's voice. Furthermore, the loud sidetone may cause the near talker to reduce his/her own voice volume to a point that the far party may not be able to hear the conversation.